1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) magnetic field sensor in a magnetic read head, more specifically to a spin valve type of GMR sensor of the top spin valve type.
2. Description of the Related Art
Early forms of magnetic read heads decoded magnetically stored data on media such as disks and tapes by making use of the anisotropic magnetoresistive effect (AMR) in magnetic materials such as permalloy. This effect is the change in the electrical resistance, r, of certain magnetic materials in proportion to the angle between the direction of their magnetization and the direction of the current through them. Since changing magnetic fields of moving magnetized media, such as magnetically encoded tapes and disks, will change the direction of the magnetization in a read head, the resistance variations of the AMR effect allows the information on such encoded media to be sensed and interpreted by appropriate circuitry.
One shortcoming of the AMR effect was the fact that it produced a maximum fractional resistance change, Dr/r (where Dr is the change in resistance between the magnetic material subjected to its anisotropy field, Hk, and the material subjected to zero field), which was only on the order of a few percent. This made the sensing process difficult to achieve with accuracy.
In the late 1980""s and early 1990""s the phenomenon of giant magnetoresistance (GMR) was discovered and soon applied to read head technology. The GMR effect derives from the fact that thin (20-80 angstroms) layers of ferromagnetic materials, when separated by even thinner (20-30 angstroms) layers of electrically conductive but non-magnetic materials, will acquire ferromagnetic (parallel spin direction of the layers) or antiferromagnetic states (antiparallel spin direction of the layers) by means of exchange interactions between the spins. As a result of spin dependent electron scattering as electrons crossed the layers, the magnetoresistance of such layered structures was found to be significantly higher in the antiferromagnetic state than the ferromagnetic state and the fractional change in resistance was much higher than that found in the AMR of individual magnetic layers.
Shortly thereafter a version of the GMR effect called spin valve magnetoresistance (SVMR) was discovered and implemented. In the SVMR version of GMR, two ferromagnetic layers such as CoFe or NiFe are separated by a thin layer of electrically conducting but non-magnetic material such as Cu. One of the layers has its magnetization direction fixed in space or xe2x80x9cpinned,xe2x80x9d by exchange anisotropy from an antiferromagnetic layer directly deposited upon it. The remaining ferromagnetic layer, the unpinned or free layer, can respond to small variations in external magnetic fields such as are produced by moving magnetic media, (which do not affect the magnetization direction of the pinned layer), by rotating its magnetization direction. This rotation of one magnetization relative to the other then produces changes in the magnetoresistance of the three layer structure.
The spin valve structure has now become the implementation of choice in the fabrication of magnetic read head assemblies. The trend in recent patents has been to improve the sensitivity and stability of these spin valves by novel choices of the materials used to form their various ferromagnetic and antiferromagnetic pinned and pinning layers and by variations in the number, positioning and dimensions of such layers. In this connection, Kanai (U.S. Pat. No. 5,896,252) teaches a method for constructing a spin valve magnetoresistive (SVMR) head element in which the free (unpinned) magnetic layer is manufactured in a two-layer structure composed of a CoFe layer and an NiFe layer. Fontana, Jr. et al. (U.S. Pat. No. 5,701,223) teach a method for forming a spin valve magnetoresistive sensor using a laminated pinned layer that is placed in a magnetically exchange-coupled antiparallel configuration and combined with an antiferromagnetic exchange biasing layer. The pinned layer comprises two ferromagnetic films that are separated by a non-magnetic exchange coupling film.
Yamada (U.S. Pat. No. 5,852,531) teaches a method of constructing a spin valve magnetoresistive head that reduces the asymmetry in head response caused by a competition between the giant magnetoresistive effect (GMR), which is due to changes in magnetization direction between the pinned and unpinned layer and the anisotropic magnetoresistive effect (AMR), which is due to variations in angle between the current and the magnetization direction of the single unpinned layer. Finally, Gill (U.S. Pat. No. 5,920,446) teaches a method of forming a GMR sensor for ultra-high density recordings by using two free layers rather than a free layer, a pinned layer and a pinning layer. This formation reduces the overall thickness of the read head, making it more suitable for decoding higher recording densities. The free layers are each laminated, comprising two ferromagnetic layers coupled in an anti-parallel configuration by a spacer layer. The sense current flowing through the formation provides the necessary bias field to set the magnetic field directions of the layers and the variations in magnetization direction in either or both layers produces the requisite resistance variations.
Further improvements in the design and fabrication of SVMR read heads must now be directed towards their use in decoding hard disks whose magnetic information content is approaching an area density greater than 35 gigabytes per square inch (35 Gb/in2). At such extreme densities, the read head requires an increasingly narrow read gap and correspondingly thinner and/or fewer magnetic layers. As the thickness of these layers decreases, however, it becomes increasingly difficult to obtain a controllable bias point, a high GMR ratio (Dr/r) and good magnetic softness. As presently fabricated, SVMR sensors, such as those referred to above, are adequate for densities on the order of a few gigabytes per square inch, but they lack the requisite physical properties to accurately decode the increased density. It is the aim of the present invention to also address the problem of fabricating an SVMR read head that is capable of decoding ultra-high densities of magnetically encoded information.
A first object of this invention is to provide a method for forming a magnetoresistive (MR) sensor element whose operation is based upon the giant magnetoresitive properties of certain magnetic structures, along with the magnetoresistive (MR) sensor element whose operation is so based.
A second object of this invention is to provide a method for forming a magnetoresistive (MR) sensor element which is capable of and suitable for decoding ultra-high density (35 Gb/in2 and above) magnetic recordings, along with the magnetoresistive (MR) sensor element having said capability and suitability.
A third object of this invention is to provide a robust spin valve magnetoresistive (SVMR) sensor with a controllable bias point, excellent thermal stability and high output performance.
In accord with the objects of this invention there is provided a spin valve magnetoresistive (SVMR) sensor and a method for its fabrication. Said spin valve magnetoresistive sensor is of the single top spin valve structure, which provides advantages in the reading of ultra-high density magnetic data because it comprises fewer layers and is correspondingly thinner than other structural forms. In addition to the improved read capabilities associated with the thinness of the single top spin valve structure, the inherent simplicity of the form leads to a simpler, more efficient and more economical manufacturing process.
Further in accord with the objects of this invention, there is provided a single top spin valve magnetoresistive sensor element (xe2x80x9ctopxe2x80x9d referring to the position of the pinned layer) for which a typical optimal configuration (as formed between an upper and lower substrate) is empirically determined to be:
X1 NiCr/X2 Ru/X3 CoFe/X4 Cu/X5 CoFe/X6 NiCr/X7 CoFe/X MnPt/X9 NiCr
where the symbols, Xn, represent thicknesses of the various materials given in a range of angstroms according to Table 1, below:
Further in accord with the objects of this invention and as can be seen from the configuration depicted above the table (in which the xe2x80x9ctopxe2x80x9d of the configuration is to the right), said top spin valve structure of the present invention is fabricated so as to employ and embody the advantages resulting from the specular reflection of conduction electrons from certain material layers of the spin valve structure, specifically from the Ru/CoFe layer combination whose thicknesses fall within the ranges specified above. Such specular reflection, when acting in concert with the spin dependent scattering of the GMR effect, produces further enhancements of the magnetoresistance ratio, Dr/r, thus making the structure able to sense the weaker magnetic signals produced by higher density recordings. Still further in accord with the objects of this invention, said CoFe layer (the free layer) grown on a Ru layer is characterized by superior thermal stability and produces extremely favorable magnetic properties, specifically improved magnetic softness and uniaxiality, resulting in higher signal sensitivity.
Yet further in accord with the objects of this invention, there is provided a laminated pinned layer structure of the form CoFe/NiCr/CoFe to insure robustness in the sense that the magnetic orientation of the pinned layer will remain essentially constant after it is oriented by annealing. Finally, in accord with the objects of said invention there is incorporated an MnPt pinning layer deposited on a ferromagnetic layer. Under such circumstances, MnPt is characterized by a high blocking temperature, high exchange bias field (Hex) and superior corrosion resistance.